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My new favorite car magazine, Automotive Design and Production, has an article about the Elise chassis that any engineer would love. :)

Here's the text from the article:

Lotus Bonds with Aluminum

By Kermit Whitfield, Senior Associate Editor


If aluminum-intensive cars are ever to become more than an occasional curiosity, automakers may have to give up their weld shops. At least that’s the conclusion you could draw after talking with the people at Lotus Engineering (Hethel, England). Lotus has been building cars with aluminum chassis for many years, but none of them are welded: they are held together with screws and adhesives. When Lotus first introduced the method on the low-volume Elise in 1996, company leaders were worried about market acceptance for what is essentially a glued-together car, but the technique proved so successful (over 23,000 cars produced with no reported failures) that it has become the basis of a new higher volume venture that may help to bring aluminum-intensive vehicles more into the mainstream. The new project, which uses Lotus’ Versatile Vehicle Architecture (see AD&P February 2004, “Lotus’ Versatile Venture”), tweaks the lessons learned on the Elise for volume production, but remains true to the fundamental concept: to get the most out of aluminum structures you must design for the material, not treat it like a steel substitute.


Bond, Adhesive Bond. First and foremost, that means not welding it. Why? “The yield strength of aluminum goes down by half once its welded,” explains Richard Rackham, vehicle architect at Lotus. So, getting the same strength in a welded aluminum chassis as in a bonded unit requires doubling the amount of material used; since aluminum is usually chosen for its light weight, that dilutes its key benefit. Another big disadvantage of welding aluminum is that stresses are localized along a point or a line, which can lead to material fatigue. Stresses are distributed over a wide part-mating when bonding is used. To gain the full merits of using adhesive, Lotus had to come up with ways to optimize its properties. For example, after determining that the optimum bond gap between parts is 0.2 mm, the question became how to maintain that gap uniformly over the bonded surface. The answer: Lotus designed tiny protrusions, or “pips” on the parts that held them exactly 0.2 mm apart.

To fabricate the main chassis components Lotus chose a process well-suited to aluminum: extrusion. Chassis supplier Norsk Hydro ASA (Oslo, Norway) extrudes the closed-box parts out of 6000 series aluminum and bonds them to folded sheets of recycled 3000 series. Rackham says that one reason extrusions were chosen is because they can be inexpensively produced (he estimates the cost of a die at about $5,000), which helps to offset the higher material costs of aluminum. Another is that they can be formed into complex shapes that serve multiple purposes and help keep parts count down. The proof: the entire Elise chassis consists of only 27 different extrusions.


Higher Volume. The challenge Lotus now faces is translating the aluminum production methods for an essentially hand-made $40,000 sports car to affordable vehicles that can be mass-produced. It’s current initiative, which is being conducted for an unnamed automaker, looks to build sub-$30,000 vehicles in the 40,000 to 50,000 annual unit range. To do that, Lotus is making some changes. According to Kerry Osborne, principal engineer, the hand-applied flow drill screws that are currently used to knit the Elise chassis together are being replaced by self-piercing rivets which can be shot more quickly, though they require application tools that generate at least five tons of pressure. But perhaps the biggest change is in the bonding. Realizing that no mass-production operation can afford the Elise’s 50-minute curing time (nor would it wish to incur the expense of multiple ovens), Lotus is replacing the heat-cured single part epoxy adhesive used on the sports car, which required temperatures of 180°C, with one that will cure in the lower heat of the paint oven. (Both adhesives are sourced from Dow Automotive [Auburn Hills, MI].) The savings in process time garnered by these changes could be enough to peak the interest of volume automakers and get them to consider bonded aluminum chassis as a viable alternative for niche vehicle production. After all, what do they have to lose, except their weld shops?
 

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The new glue ?

"Dow Automotive is a leading supplier of plastics, adhesives, sealants, plastics-enhanced products, and body engineered systems to the global automotive industry, serving tier, OEM and aftermarket customers. With offices and application development centers worldwide, the organization provides material science expertise and comprehensive technical capabilities to its customers worldwide.
With BETAMATE™ structural adhesives you can replace welds and mechanical fasteners in joining a variety of similar as well as dissimilar substrates. And using BETAMATE adhesives both reduces fatigue and failure commonly found around spot welds and fasteners, and also seals against corrosion causing environmental conditions. Acoustics may also be improved because BETAMATE reduces vibration by stiffening the overall vehicle structure."

It could be Betamate XW1185 an epoxy based adhesive which cures at room temperature.
But is it strong enough?
Other Betamates are PU based or need special ovens and time for curing
 

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Sorry but perhaps you had no time to read this:

“But perhaps the biggest change is in the bonding. Realizing that no mass-production operation can afford the Elise’s 50-minute curing time (nor would it wish to incur the expense of multiple ovens), Lotus is replacing the heat-cured single part epoxy adhesive used on the sports car, which required temperatures of 180°C, with one that will cure in the lower heat of the paint oven. (Both adhesives are sourced from Dow Automotive [Auburn Hills, MI].)”

The actual glue is made by Ciba
 

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PS

Possibly American glue would help you to get your cars earlier but you never know whether it will be the glue which will stick that long. :rolleyes:
 

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Wasn't there some article earlier quoting Lotus engineers as saying they haven't found any rivets that work as well has hand-applied screws for a long-lasting bond? Yikes.

I'm not sure I want to be hearing about changes to the time-tested chassis bonding to speed up the build process.
 

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It's always been my understanding that the rivets and screws "used to hold the chassis together", are not for structural purposes. They are there to hold the parts in position until the glue bonds - the rivets are simply alignment fixtures.

Although I constantly read references to the Elise's bonded and riveted chassis in all the magazines...:rolleyes:

Tim Mullen
 

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The rivets also provide protection in the event of an accident. The glue is extremely strong in shear, but the panels can be "peeled" apart and the rivets are there to prevent this from happening.
 

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The new bonding agent curing below 200°C is Betamate 4601 from Dow Chemicals. A friend gave me a cartridge to play with - it is quite amazing stuff which does indeed not need a lot of heat to cure. OTH it is important to up the temperature really slowly, so the curing process remains a time-consuming job (heat-up phase from room temperature is about 30 minutes, then 15 minutes curing at steady 175-180°C). Don't think of your wife's kitchen oven now - the smell is :rolleyes:.

Ruediger, especially for you:
http://www.aws.org/wj/supplement/06-2003-MESSLER-s.pdf
:D
 

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Here in Germany Lotus might have a problem with the TÜV when using new glue and rivets because they would have to prove by tests that the quality of the chassis structure is not affected by those changes in technology. Somebody over there knows how this is handled in the USA ?
 

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Stefan said:
The new bonding agent curing below 200°C <...> then 15 minutes curing at steady 175-180°C).
That's still a long way from "room temperature" curing...

Tim Mullen
 
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